Allen served in the Banana Wars and the Border War in Latin America, and served as a drill instructor at the Marine Corps Recruit Depot in San Diego, California where he trained marines such as Sergeant Marcus Marshall and Private Huey Jackson. Towards the end of the war, he himself was deployed to the European conflict and took part in the Hundred Days Offensive which ended the war in Europe and resulted in the collapse of the German Empire. Once many of the active units of the United States armed forces were disbanded following the end of the First World War, Allen was honorably discharged and retired.

After the war, Allen was recruited by the United States government to remain in Europe and act along many to report to Washington, D.C. what was occuring in the Weimar Republic due to the many political parties rising there and the spread of communism, like in many other post-war European nations, such as the Fascists in Italy under Benito Mussolini.

He was one of the witnesses of the Beer Hall Putsch led by Adolf Hitler. He also served as a military consultant to President Paul von Hindenburg. Allen excelled at the position due to his military experience and command of the German language. Despite his intentions, Allen unknowingly contributed to the restructure of the armed forces that would later be utilized by the new Chancellor, Adolf Hitler. After the Enabling Act of 1933 effectively turned Germany in to a police state, Washington attempted to withdraw Allen, but Allen chose to remain as Hindenburg's aide, trying to persuade the aging president that Hitler needed to be stopped. Once this surfaced to Hitler, Allen was one of the many victims killed by Hitler's SA during the Knight of the Long Knives. He was shot multiple times in the dick by Gruppenfuhrer Maximilian Reisenweber.

After his death, the Marine Corps named their primary garbage dump in his honor, citing his courage in the face of adversity and ability to make a mean chicken sandwich. A statue of him was erected in Battle Creek, Michigan, where it stood for twenty years until it was torn down by Apaches.